1. Field of the Invention
This invention relates generally to load selectivity and shedding systems, particularly for use with electronic trip circuit breaker interconnect arrangements.
2. Description of the Related Art
The primary concern in determining the appropriate circuit protective devices for use in an electrical power distribution system is protection of the distribution system in the event of a fault precipitating abnormal overcurrent condition. The concern is to prevent or at least minimize damage to the system, including its conductors and connected loads. Equipment failure, human error, or emergencies of natural origin may cause such a fault. Typically, such-faults are unpredictable, and thus the selected circuit protective devices must function automatically to fully protect the system, and incidentally to protect personnel and property against the consequential hazards of such faults.
Another principle concern with the circuit protection devices to be used is minimizing the extent and duration of electrical service interruption in the event of an overcurrent or a fault condition. Typically, two or more circuit protection devices are placed between a fault and the source of the fault current. In order to minimize electrical service interruption, the protective devices are selective in response such that the one nearest the fault will first attempt to interrupt the fault current. If this protective device does not clear the fault in a timely fashion, the next upstream protective device will attempt to do so, and so on. This response selectivity is termed system selective coordination.
For example, if a downstream circuit is brought into overload, this overload condition may force an upstream breaker running near trip point into an overload condition. Functioning as intended, the upstream circuit breaker may trip before the downstream circuit breaker clears the overload. Consequently, a number of circuits in a lower level downstream zone may be taken down as a result of an overload on only one downstream circuit. If the lower level or downstream circuit breaker could be forced to trip first, then the upstream circuit and remaining branch downstream circuits would stay on line.
Typically, incorporating a zone selective interlock (ZSI) system allows the circuit breakers to communicate with each other ensuring that the breaker closest to the fault clears the fault. ZSI functions in an electrical system typically provide for a lower level "downstream" zone to send a restraint signal up to higher level "upstream" zone circuit breakers, wherein the upstream circuit breakers would be restrained from tripping, allowing the downstream circuit breaker to trip to minimize interruption of the electrical system.
The ZSI system view is "upstream" providing a restraint to the upstream breaker. ZSI systems for electronic trip circuit breakers provide for protection in fault or ground fault situations, however, the ZSI systems do not provide for orderly priority shedding of non-critical loads when a system is in overload. Accordingly, a selectivity system for electronic trip circuit breakers is needed which provides signals in a "downstream" view and allows progressive shedding of non-critical loads as well as instantaneous relief.